Cellular communication systems are being developed and improved for machine type communication (MTC), communication characterized by lower demands on data rates than, for example, mobile broadband, but with higher requirements on, for example, low cost device design, better coverage, and an ability to operate for years on batteries without charging or replacing the batteries. In the 3GPP GERAN specification group, cellular communication systems are being improved and developed in the feasibility study named “Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things.” GSM is being evolved, and new “clean slate” systems (systems not based on current cellular systems) are being developed.
One “clean slate” approach, called narrowband machine-to-machine (NB M2M), is a narrowband system with a carrier bandwidth of 200 kHz that targets improved coverage compared to GSM systems, long battery life, and low complexity communication design. In cellular communication systems, devices use a cell search procedure (or synchronization procedure) to understand which cell(s) to connect to. The essential functions of a cell search procedure include detecting a suitable cell to camp on, and for that cell, obtaining the symbol and frame timing and synchronizing to the carrier frequency. When synchronizing to the carrier frequency, the mobile station needs to correct any erroneous frequency offsets that are present, and perform symbol timing alignment with the frame structure from the base station. In addition, in the presence of multiple cells, the mobile station also needs to distinguish the particular cell on the basis of a cell ID, and obtain the corresponding frame number to perform frame synchronization. Thus, a typical cell search procedure consists of determining the timing alignment, correcting the frequency offset and obtaining both the correct cell ID as well as the frame ID.
When the device wakes up from deep sleep, for example from being in a power saving state, the frequency offset is to a large extent due to device clock inaccuracy (often assumed to be up to 20 ppm). The clock inaccuracy results in a drift in the timing of the sampling of the received signal. To the device receiver, this drift appears mainly as a frequency offset of the received signal, a continuous rotation of the received samples. For a system operating with 20 ppm with a carrier frequency of 900 MHz, the maximum frequency offset is 18 kHz. This offset needs to be estimated and corrected for.
The cell search procedure for NB M2M is described in GP-140864, “NB M2M—Cell Search Mechanism” and GP-140861, “NB M2M—Frame Index Indication Design.” As described in GP-140864, cell search is assumed to be performed using three sequences:                (a) Primary Synchronization Sequence (PSS): The PSS is used to determine the frame timing alignment, along with a coarse estimation of the frequency offset.        (b) Secondary Synchronization Sequence (SSS): The SSS is used to obtain a finer estimate of the frequency offset. Together with the PSS, the SSS also determines the cell ID.        (c) Frame Index Indication Signal (FIIS): The FIIS is used to determine the frame number (i.e., the ID of the current frame in the superframe). Each superframe consists of 64 consecutive frames.Every frame consists of 960 symbols. 256 symbols are dedicated to PSS, 257 for SSS, 127 for FIIS, and the remaining 320 symbols are for carrying the broadcast information in a Broadcast Information Block (BIB).        
The combination of PSS and SSS is also used to determine the ID of the particular cell after the MTC device has performed the timing and frequency synchronization. In order to achieve this functionality, three pre-defined sequences are used for PSS, and twelve are used for SSS, giving a total of thirty-six possible combinations. Each combination is used by a particular cell. This, in turn, enables the MTC device to determine the cell ID. Specifically, the MTC device first tests each of the three PSS to determine the one with the highest correlator output. This gives the frame timing. Then, the device tests each of the twelve SSS to determine the one with the highest output at the correlator to correct the frequency offset. Once the two sequences have been found, they correspond to one of the thirty-six possible combinations, which determines the cell ID. The next sequence, FIIS, is then used to obtain the frame number. This completes the cell synchronization procedure.
After switching on, an MTC device first needs to search for a signal in a viable frequency band. Signal detection is performed on the basis of comparing the amplitude of the peak from a correlation based detector with a pre-determined threshold. This is achieved by correlating the received signal with a known sequence, or a set of known sequences.
GSM uses another method for cell search. In GSM, first the frequency offset is estimated without knowledge of timing, then the time offset is estimated. This is done with signals that are specifically designed for that order of operation. Another difference is that, in GSM, the synchronization signals from different base stations use different frequency resources, which is a different way of handling multiple cells and cell IDs.
In the NB M2M cell search procedure, a physical channel named Physical Broadcast Synchronization Channel (PBSCH) is dedicated to carrying the synchronization signals, along with the broadcast system information. A separate downlink physical channel per base station is reserved for PBSCH, while the data channels are multiplexed by frequency division multiplexing (FDM). In addition, the PBSCH operates with a reuse factor of 1, implying that the PBSCH of neighboring cells are completely overlapped in the frequency domain. This has the advantage of a reduction in search complexity, but also results in interference from all the other cells using the PBSCH.